This invention is related to techniques for fabricating high temperature superconducting oxide materials, and more particularly, to techniques for making thin films of such materials by metalorganic chemical vapor deposition (MOCVD).
The discovery of superconducting oxide materials with critical temperatures above the boiling point of liquid nitrogen has stimulated much interest in methods for making thin films of these materials. Such films can be extremely useful in electronic devices and energy transport systems, and many researchers have devoted substantial effort to finding a satisfactory method of fabricating these films.
Chemical vapor deposition has been extensively used for preparation of films and coatings in a variety of applications. The advantages of this method include higher quality, faster processing and the ability to coat substrates of irregular shapes. Accordingly workers in the relevant field have been attempting to fabricate superconducting oxide films by chemical vapor deposition in a feasible manner. In particular, much attention has been devoted to the growth of thin films of the celebrated Y-Ba-Cu-O material by MOCVD. Such films were first fabricated in 1988, within the year after the discovery of the superconducting properties of YBa.sub.2 Cu.sub.3 O.sub.7-x.
The primary difficulty with this technique arises from the solid precursor reagents that are used as the source materials. Typically these metalorganic precursors are tetramethylheptanedionate (TMHD) powders that are chelates of the Y, Ba, and Cu source metals. These precursors tend to decompose at temperatures close to the vaporization temperatures. This is particularly true of the Ba-TMHD precursor, which decomposes almost as fast as it vaporizes. Thus the quantity and volatility of the sources change continuously and non-reproducibly. Therefore it is important to control precisely the exposure of these precursors to elevated temperatures. Ideally this exposure should be as short as possible; however the chemical vapor deposition method requires that the precursor sources be stabilized at the vaporization temperatures, which means that this exposure may extend over a substantial period of time. Some workers have succeeded in fabricating high quality Y-Ba-Cu-O films, despite these obstacles, but low deposition rates are reported, typically 1 .mu.m/h. In addition, the control of the process is very delicate and repeatability is poor. The commercial feasibility of these previous processes has not been established heretofore.
The standard chemical vapor deposition method employs precursor sources in separate vaporization chambers. The precursors, which may be solid or liquid, are placed into boats or bubblers, and individually heated to a temperature at which they develop appreciable vapor pressures. The vaporized precursor material is transported by passing a carrier gas over the boat or bubbler, and sweeping the vapors through a gas jungle to the reaction chamber where they are mixed and the reaction product is deposited on the heated substrate. This technique works well for liquid precursors, where the carrier gas can be bubbled up through the liquid reservoir to fully saturate it before entering the gas jungle. In this way, the amount of material transported to the reaction chamber is precisely controlled by the temperature of the precursor reservoir (which must be in thermal equilibrium) and by the flow rate of the carrier gas bubbled through the reservoir. Elaborate schemes have been developed to ensure complete saturation, precise temperature control, and accurate gas flow regulation. When this method is used to grow heterostructure films with sharp boundaries between layers, the pressures of the various gas streams must be balanced to avoid composition excursions when the composition is changed.
This standard technique encounters further problems when it is used for solid-state precursors. It is difficult to ensure complete saturation of the carrier gas stream at the elevated temperatures required to raise the solid vapor pressure to appreciable values because the surface area of the solid is changing continuously due to depletion and grain growth effects. This problem may be alleviated by using large excesses of precursor material beyond the amount needed for film growth. This can result in a substantial waste of precursor material.
The use of the conventional MOCVD method for making superconducting oxide films is discussed in the article "Preparation of Superconducting-Oxide Films by CVD and Their Properties" by H. Yamane, H. Kurosawa and T. Hirai, published in Journal de Physique, Colloque C5, Supplement au n.degree. 5, Tome 50, Mai 1989. This paper describes the parameter control required to grow these films and the resulting properties of films made in this way. The problems discussed above are referred to.
The decomposition problem is also addressed in the article "Rapid Chemical Vapor Deposition of Superconducting YBa.sub.2 Cu.sub.3 O.sub.x " by W. J. Lackey, W. B. Carter, J. A. Hanigofsky, D. N. Hill, E. K. Barefield, G. Neumeier, D. F. O'Brien, M. J. Shapiro, J. R. Thompson, A. J. Green, T. S. Moss, III, R. A. Jake, and K. R. Efferson, published in Applied Physics Letters, 56 (12), 19 Mar. 1990, pages 1175-1177. These authors describe a powder feed method for introducing mixtures of powdered reagents into the carrier gas, which itself is a mixture of argon and oxygen. The powder is transported to the reaction zone where it vaporizes, reacts, and deposits YBa.sub.2 Cu.sub.3 O.sub.x on a hot substrate. This method achieves substantially higher deposition rates and improved film quality control in comparison with the standard separate-source method. The article also reports improvements with respect to the process control problem.